The present invention relates to the use of an electrode system, comprising a measurement electrode, a reference electrode and a counterelectrode, for measuring the concentration of hydrogen peroxide. The present invention also relates to a method for measuring the concentration of hydrogen peroxide in a solution.
With regard to the state of the art, reference is made to Finnish Patent No. 80,526 (corresponding to U.S. Pat. No. 4,933,292, the specification of which is incorporated by reference herein) which describes a method for controlling the pulping process. In the method described in this patent, an electrode system is used which comprises one or more measurement electrodes, reference electrodes and power supplying counterelectrodes, positioned in a pulp boiler. Current is supplied from a power source to the circuit so that the voltage between the measurement electrode and the reference electrode, in other words, the electrochemical potential of the measurement electrode is substantially constant, so that the current equivalent to this potential is directly proportional to the activity of the chemicals in the boiler. For the measurement electrode, coal steel, iron, copper, zinc, cadmium, or monel metal can be used. The electrochemical potential of the measurement electrode varies in the range from about -500 millivolt to about -1500 millivolt to the calomel electrode.
The concentration of hydrogen peroxide is required to be measured, inter alia, from the bleaching pulp. A change in the hydrogen peroxide concentration influences a variety of factors in the solution. Such factors are, among other things, the idle potential of the measurement electrode, the current densities measured on the polarization curve, and the zero point of the polarization curve, pH, the conductivity of the solution, and temperature.
The difference of potential between the measurement electrode and the electrolytic solution, when measured in relation to the reference electrode, i.e., the idle potential of the measurement electrode, changes as a function of the hydrogen peroxide content in the solution. The direction and intensity of the change is dependent on the electrode material used. When an inert material, such as platinum, is used for the measurement electrode, the measurement mentioned above yields a so-called redox potential. The redox potential is a potential difference characteristic of the solution, and caused by redox reactions on the surface of the electrode, thereby measuring the oxidation capacity of the solution The redox potential behaves when compared with the idle potential of an electrode made of a less noble material, less actively because the dissolving metal ion is non-existent.
An electrode arrangement according to Finnish Patent No. 80,526 is used in which a platinum electrode is used as the measurement electrode for measuring the hydrogen peroxide content in a solution. However, the use of a platinum electrode for the measurement electrode provides a relatively inaccurate dependency on the hydrogen peroxide concentration because in the polarization curves, it can be seen that a change in the current density divided by a change of the concentration, i.e. the ratio .DELTA.I/.DELTA.C, is too low for accurate measurement of the hydrogen peroxide concentration. In addition, the soilings of the electrodes generate inaccuracy in measurement results.